Very often, especially in automation technology, position-measuring devices are needed to monitor linear and rotational movements. Linear movements are monitored using so-called length-measuring devices, whereas rotational movements are monitored using so-called rotary encoders or angle-measuring devices.
Rotational movements are mostly produced by electric motors which drive a motor shaft that moves machine parts of technical equipment, possibly via clutches and transmissions. In order to be able to perform accurate positioning operations, electric motors are operated in control loops; one speaks here also of servo drives. Actual position values, which are required for feedback control, are continuously measured by monitoring the angular position and/or the number of revolutions of the shaft of a position-measuring device (rotary encoder or angle-measuring device).
EP 1 126 248 A2 describes a position-measuring device that is suitable for this purpose. This position-measuring device includes a measuring graduation on a graduation carrier that is non-rotatably connectable to the shaft, so that the measuring graduation rotates in a measuring graduation plane which is perpendicular to the axis of rotation of the shaft. To permit scanning of the measuring graduation, a scanning unit is disposed opposite the same during operation, the scanning unit being disposed in a scanning plane parallel to the measuring graduation plane. The distance between the planes is referred to as scanning distance. During assembly of the position-measuring device, this distance is set as precisely as possible to achieve an optimum operating condition.
In addition to rotational movements, it is often also necessary to measure linear movements of machine parts that move in the axial direction of the motor shaft. This is the case with braking devices, for example. These serve to allow an electric motor to be quickly braked and brought to a standstill in the event of a failure. To do this, a machine part supported so as to be movable in the axial direction of the shaft is pressed with a force against a brake disk that is non-rotatably connected to the shaft. This contact causes friction, which produces a braking effect. In order to optimize the braking process, a brake lining is disposed in the region where the friction occurs. Since the brake lining is abraded over time, it is desired to monitor the thickness of the brake lining so as to be able to replace it in time during a maintenance operation. The thickness of the brake lining can be inferred, for example, by measuring the distance that the movably supported machine part must travel in order to activate the brake. This can be done using a length-measuring device. However, this involves considerable structural complexity and is also expensive.